I don't know if anyone mentioned on this thread the reason for posting on diagonals? I mean, WHY do we rise with the outside shoulder's forward swing?
Ok, here's what I learned;
When we are on a circle (which is the only time being on the "correct" diagonal matters) , as the outside fore is going forward, the outside hind is pushing off and moving backwards (they move apart from each other). At the moment that the outside hind is pushing off the ground, it make the horse's hip be higher. If the rider goes up right at the horse is pushing off , this both takes advantage of that push and makes it easier for the horse, by not inhibiting it's full movement. On a circle, the outside legs must move farther than the inside, so rising up with the outside push makes sense.
This is what I was told. Fact? I dont' know. Heck, half the stuff we learn is what we are told by someone else. But I know that it usually feels better when posting on the correct diagonal, certainly when going around a circle.
On a straight line, I dont' think it matters.